1. Field of the Invention
The present invention relates to the biological control of several weed species by the use of a pathogenic toxin from a strain of the fungus Alternaria alternate.
2. Description of the Prior Art
Members of the genus Alternaria are known to produce a wide range of phytotoxic compounds which affect a large number of the plants on which the fungus is found (Bruce, V.R., Stack, M.E., and Mislivec, P.B. [1984] J. Food Sci. 49:1626-1627; Harvan, D.J., and Pero, R.W. [1976] J.V. Rodricks, ed. Advances in Chemistry, Series 149. pp 344-355). These phytotoxins include alternariol, alternariol monomethylether (AME), altenuene, altenuic acid, tenuazonic acid (TA), tentoxin, alternaric acid, AK-toxin, and AAL-toxin, and possess a broad range of biological activities and metabolic effects (Bruce et al., supra; Harvan et al., supra; Nishimura, S., and Kohmoto,K. [1983] Annual Rev. Phytopathology 21:87-116).
These phytotoxins have been referred to as `host-specific` because they "are toxic only to the host that is susceptible to the pathogen which produces the toxin, and if they induce nearly all symptoms of the disease are considered to be definitive chemical probes in the study of disease susceptibility and physiological stress at the molecular level" (Bottinii A.T., and Gilchrist, D.G. [1981] Tetrahedron lett. 22:2719-2722). "Physiological, biochemical, genetic, and histological data all confirm that these toxic compounds are the key determinants of disease and of host selection by the producing fungi" (Scheffer, R.P. [1989] Kohmoto, K. and Durbin, R.D., ed. Host-Specific Toxins, pp. 1-17).
Furthermore, it is believed by phytopathologists that tolerance and sensitivity to a toxin is controlled by the same genes in the same way that they control resistance and susceptibility to the fungus (Scheffer, Robert P. and Livingston, Robert S. [1984] Science 223:17-21). If a fungus does not grow on a plant, the phytotoxin produced by the fungus will not affect that plant, depending on whether or not it has dominant or recessive alleles (Grogan, R.G., Kimble, K.A., and Misaghi, I. [1975] Phytopathology 65:880-886). In general, resistance in cultivare is equivalent to insensitivity to a metabolite released by the pathogen (Scheffer, R.P., supra). It has been suggested that these host-specific toxins are suitable for use as tools for screening resistant genotypes in plant breeding programs (Clouse, S.D., and Gilchrist, D.G. [1987] Phytopathilogy 77:80-82; Clouse, S.D., Martensen, A.N. and Gilchrist, D.G. [1985] Journal of Chromatography 350:255-263; Scheffer et al., supra).
Even in a single species there may be a variety of strains which will be morphologically the same but can produce different toxins to which different hosts are susceptible. These are called pathotypes. A. alternata includes many pathotypes which are disease-producing in specific plants (Nishimura et al., supra; Stierle, A.C., Cardellina II, J.H., and Strobel, G.A. [1989] J. Natural Products 52:42-27).
One of these pathotypes is A. alternate f. sp. lycopersici which causes a serious stem canker disease affecting the leaves, stems, and fruits of susceptible tomato cultivars. However, the infection is unpredictable in that there are other tomato varieties which are unaffected by either the fungus or its toxin.
The structure of a host-specific phytotoxin responsible for stem canker disease has been elucidated. It was shown to be a long-chain ninhydrin-positive polyol called AAL-toxin (or Al-toxin) (Bottini, A.T., Bowen, J.R., and Gilchrist, D.G. [1981] Tetrahedron lett. 22:2723-2726; Bottini, A.T., and Gilchrist, D.G. supra). It has also been shown that in tomato cultivars which are susceptible to AAL-toxin, the pair of alleles involved is at the asc locus. The plants had three significantly different levels of toxin sensitivity, which were inherited as an incomplete dominant and corresponded to the genotype at the asc locus (Clouse et al. [1987] supra).
Apparently, however, not all stem canker disease is caused by AAL-toxin. A stem canker causing phytotoxin isolated from an A. alternata f. sp. lycoversici phytotype has been reported which did not react with ninhydrin indicating the absence of primary or aromatic amines. The toxin was effective on EarlyPak Tomatoes, but not on jimsonweed (Datura stramonium) or other solanaceous species tested (Gilchrist, D.G. and Grogan, R.G. [1975] Phytopathology 66:165-171).
There are many fungi which are pathogenic to weeds and which produce phytotoxins that could be useful as herbicides (Abbas, H.K., Boyette, C.D., Hoagland, R.E., and Vesonder, R.F., [1991] Weed Sci. 39:673-677;Boyette, C.D. [1986] Plant Sci. 45:223-228; Boyette, C.D., Weidemann, G.J., Te BeeBt, D.O. and Quimby, Jr., P.C., [1991] Weed Science 39:678-681); Stierle, et al., supra). Fusarium spp. are particularly plentiful throughout the world. An isolate of Fusarium moniliforme, obtained from infected jimsonweed, was found to produce fumonisin phytotoxin (Abbas, et al., supra). Fumonisin, while structurally similar to AAL-toxin, is obtained from a species known to have a broad host spectrum, whereas A. alternata f. sp. lycoversici is host-specific.